When measuring electric potentials, for example, on the skin of a patient, the useful signal contained in these potentials lies only in the mV range, as may be the case in an electrocardiogram (ECG), an electromyogram (EMG), an electroencephalography (EEG) or an electrooculography (EOG). As such the following problems arise.
Since the body of the patient is surrounded by electric fields, potentials are formed due to capacitive coupling on the skin of the patient. This effect can generally be described in such a way that the body is coupled capacitively especially to a 230V/50 Hz alternating voltage field which is caused by power supply sources located in the surrounding area of the patient. For the sake of safety, it is, however, not allowable to couple the patient directly to a uniform surrounding ground, because this would cause a considerable risk to the patient.
In addition, likewise for the sake of safety, a measuring device, to which the electrodes on the skin of the patient are connected, must also be galvanically separated from a surrounding ground. This in turn implies that the measuring device is also coupled with its internal ground capactively to the surrounding area, so that the problem arises that the device ground lies at a potential, whose level is not known, and which generally differs from the potential of the patient.
In order to at least achieve that the patient and the ground of the measuring device lie on the same potential or at least that there be a fixed potential difference present between both, it is known to connect the device ground and the body of the patient to one another via an additional electrode.
Since, however, the device ground and the patient may generally lie on a different potential because of the inhomogeneity of the surrounding fields, which arises from the different capacitive coupling to the surrounding area, an equalizing current flows. This leads to a so-called common mode signal because of the impedance of the coupling to the patient via the electrodes, which is amplified by the amplifiers in the measuring device. When the useful signals, actually to be detected with the measurement, are very small, the common mode signal leads to the actual useful signal no longer being able to be resolved. Moreover, the difficulty arises that the amplifiers must have a high input dynamic range, so that the useful signal and the higher common mode signal overlaying this can be processed. Furthermore, a digital electronic analyzing unit arranged downstream must have a high number of bits per measured value to be able to process the large signals.
For this purpose, it is known from Bruce B. Winter et al., Driven-Right-Leg Circuit Design, IEEE Transactions on Biomedical Engineering, Vol. BME-30, No. 1, January 1983, to apply a potential, which corresponds to the mean value of the signals detected at the measuring electrodes, wherein this mean value signal is also amplified, to the additional electrode arranged on the patient by the measuring device. In order to suppress the common mode signal significantly in this way, high amplifications are necessary for the mean value signal, which is difficult to achieve. In particular, the problem arises that oscillations in the outputted signal occur in high level amplifications.
In addition, it is, further, known from DE 29 26 165 A1, on which the present invention is based, to subtract the mean value of the signals, which are sent by the amplifiers, from the input signals of the amplifiers. However, the problem here is that the common mode signal is not amplified, but rather is sent together with the useful signal at the output of the amplifier. When the useful signal is extremely small, this may cause the level of the common mode signal and that of the amplified useful signal to be on the same order of magnitude, so that these cannot easily be separated from each other. In addition, there is the problem that the amplifiers and an electronic analyzing unit arranged downstream must be adapted to also further process the comparatively large common mode signal.